essence MD5 for still images

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essence MD5 for still images

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Hello



Somewhere I discovered that ffmpeg will create essence MD5s for supported
still image formats.  I began using the following command to verify that
the image essence did not change in photos after their IPTC metadata is
revised.  Because there is no documentation of such a use case on the
ffmpeg site nor anywhere else on the web I have begun to doubt that this is
a valid use of ffmpeg or this command.  Is anyone else doing this?  Even if
you aren’t I would appreciate any comments or observations that you care to
share.



ffmpeg -i INPUT -map 0:v -f md5 OUTPUT.md5



Thank you

Kevin Irelan
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Re: essence MD5 for still images

Carl Eugen Hoyos-2
2018-01-06 20:38 GMT+01:00 *** <[hidden email]>:

> Somewhere I discovered that ffmpeg will create essence
> MD5s for supported still image formats.

And audio and video.
(From FFmpeg's pov, an image format is a video format.)

> I began using the following command to verify that
> the image essence did not change in photos after
> their IPTC metadata is revised.

> Because there is no documentation of such a use case
> on the ffmpeg site nor anywhere else on the web I have
> begun to doubt that this is a valid use of ffmpeg or this
> command.

Different possibilities exist within FFmpeg to calculate
checksums, the FFmpeg regression tests are a usage
example.

> Is anyone else doing this?

Many people are.

> Even if you aren’t I would appreciate any comments or
> observations that you care to share.
>
> ffmpeg -i INPUT -map 0:v -f md5 OUTPUT.md5

This command line will calculate the md5sum of the first
video stream in the input file.
You may want to look at the framemd5 muxer for other
use-cases, other algorithms also exist.

Carl Eugen
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Re: essence MD5 for still images

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On the same subject, when calculating the MD5 of an audio WAV file ffmpeg
defaults to 16 bit.  If the input is 24 bit it is my understanding that
it needs to be explicitly reflected in the command as -c:a pcm_s24le
otherwise the resulting MD5 will not be accurate.  Are there comparable
variations in the way ffmpeg processes still image files of different
formats, bit depths, etc., that need to be accounted for in the command
that is used?

On Sat, Jan 6, 2018 at 4:31 PM, Carl Eugen Hoyos <[hidden email]> wrote:

> 2018-01-06 20:38 GMT+01:00 *** <[hidden email]>:
>
> > Somewhere I discovered that ffmpeg will create essence
> > MD5s for supported still image formats.
>
> And audio and video.
> (From FFmpeg's pov, an image format is a video format.)
>
> > I began using the following command to verify that
> > the image essence did not change in photos after
> > their IPTC metadata is revised.
>
> > Because there is no documentation of such a use case
> > on the ffmpeg site nor anywhere else on the web I have
> > begun to doubt that this is a valid use of ffmpeg or this
> > command.
>
> Different possibilities exist within FFmpeg to calculate
> checksums, the FFmpeg regression tests are a usage
> example.
>
> > Is anyone else doing this?
>
> Many people are.
>
> > Even if you aren’t I would appreciate any comments or
> > observations that you care to share.
> >
> > ffmpeg -i INPUT -map 0:v -f md5 OUTPUT.md5
>
> This command line will calculate the md5sum of the first
> video stream in the input file.
> You may want to look at the framemd5 muxer for other
> use-cases, other algorithms also exist.
>
> Carl Eugen
> _______________________________________________
> ffmpeg-user mailing list
> [hidden email]
> http://ffmpeg.org/mailman/listinfo/ffmpeg-user
>
> To unsubscribe, visit link above, or email
> [hidden email] with subject "unsubscribe".
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Re: essence MD5 for still images

Carl Eugen Hoyos-2
2018-01-09 22:25 GMT+01:00 *** <[hidden email]>:
> On the same subject

Appears to be a different subject, but most of all, please
remember not to top-post on this mailing list.

> when calculating the MD5 of an audio WAV file ffmpeg
> defaults to 16 bit.

No, ffmpeg does not default to "16 bit" or a particular
encoder, some muxers default to pcm_s16le for audio
if you do not specify a particular encoder.

> If the input is 24 bit it is my understanding that
> it needs to be explicitly reflected in the command
> as -c:a pcm_s24le otherwise the resulting MD5 will
> not be accurate.

So, yes, if you need accurate md5 for 24 bit input,
you have to specify a 24 bit encoder.

> Are there comparable variations in the way ffmpeg
> processes still image files of different formats, bit
> depths, etc., that need to be accounted for in the
> command that is used?

Typically not:
The rawvideo (and the wrapped_avframe) encoder
accepts any pix_fmt while the pcm_s16le encoder
only accepts s16 input, the pcm_s24le encoder only
supports s32 (sic!).

Carl Eugen
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